Optical non-reciprocal phase shifters are useful in a variety of devices ranging from optical isolators used in telecommunications to optical gyroscopes. A non-reciprocal phase shift is a slight change in the velocity of propagation in the presence of a transverse magnetic field. This change reverses sign if either the field or the direction of light propagation is reversed. A non-reciprocal phase shifter introduces a predeterminable phase shift into an optical signal traveling in one direction but an equal phase shift of opposite sign to a signal traveling in the opposite direction.
Conventional magneto-optic isolators typically use Faraday rotation in magneto-optic materials, such as Bismuth-Yttrium Iron Garnet, to rotate the plane of polarization of linearly polarized light. This rotation is achieved by passing light through a magneto-optic body with a magnetic field parallel to the direction of light propagation. However, magneto-optic materials are expensive, and the introduction of a magneto-optic body into an optical fiber circuit presents critical alignment problems between the fiber input and the fiber output. Such a body also precludes interconnection by fusion of homogeneous materials. Accordingly, there is a need for a non-reciprocal optical fiber phase shifter which can be made of inexpensive optical fiber.